The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating a micromachine package.
Micromachine sensing elements (hereinafter micromachines) are well known. A micromachine includes a miniature moveable structure, such as a bridge, cantilevered beam, suspended mass, membrane or capacitive element, which is supported over a cavity formed in a silicon wafer. Since the operation of the micromachine depends upon the moveability of the miniature moveable structure, it is critical that the package, which includes the micromachine, does not contact the miniature moveable structure in any manner.
FIG. 1 is a cross-sectional view of a structure 8 during the formation of a plurality of micromachine packages in accordance with the prior art. As shown in FIG. 1, a silicon wafer 10 includes a plurality of micromachine chips 12. Micromachine chips 12 include micromachine areas 14 formed in an upper surface 10U of wafer 10. Micromachine areas 14 include the miniature moveable structure of the micromachine. Micromachine chips 12 further include bond pads 16 on upper surface 10U of wafer 10. Bond pads 16 are connected to the internal circuitry of micromachine chips 12.
Micromachine chips 12 are integrally connected together in an array format. Each of micromachine chips 12 is delineated by a singulation street 20 which is located between adjacent micromachine chips 12.
A silicon lid 30 formed from a silicon wafer is positioned above wafer 10. Lid 30 includes a plurality of caps 42 integrally connected to one another. Each cap 42 includes a micromachine cavity 32. Each micromachine cavity 32 is positioned over a corresponding micromachine area 14. Generally, micromachine cavities 32 are wider than micromachine areas 14.
Each cap 42 further includes a bond pad cavity 34. Each bond pad cavity 34 is positioned over a corresponding set of bond pads 16 on a micromachine chip 12. Generally, bond pad cavities 34 are wider than bond pads 16, and are at least as deep as bond pads 16 are tall.
FIG. 2A is a cross-sectional view of structure 8 of FIG. 1 at a further stage in fabrication in accordance with the prior art. As shown in FIG. 2A, lid 30 is attached to wafer 10. Micromachine cavities 32 are positioned above corresponding micromachine areas 14. Further, bond pad cavities 34 are positioned above corresponding sets of bond pads 16.
FIG. 2B is a cross-sectional view of structure 8 of FIG. 2A at a further stage of fabrication in accordance with the prior art. Referring to FIG. 2B, a series of shallow cuts are made to remove a portion of each cap 42 to expose bond pads 16. Micromachine chips 12 are electrically tested by connecting test probes to bond pads 16. Should testing of a micromachine chip 12 indicate that the micromachine chip 12 is defective, the micromachine chip 12 and/or corresponding cap 42 is marked. For example, micromachine chip 12A is marked as being defective. Wafer 10 is then singulated along singulation streets 20. Micromachine chips 12 which are marked as defective are discarded.
Disadvantageously, a cap 42 is attached to a micromachine chip 12 even if the micromachine chip 12 is defective. The cap 42 and defective micromachine chip 12 are discarded. However, since a cap 42 is attached to the defective micromachine chip 12, the cost associated with the defective micromachine chip 12 is increased compared to the cost associated with the defective micromachine chip 12 alone. This increase is the cost of fabricating each batch of micromachine packages. This, in turn, increases the cost of fabricating each individual micromachine package which passes testing.
After singulation of wafer 10, each good micromachine chip 12 with cap 42 is further package FIG. 3 is a cross-sectional view of a single micromachine package 40 in accordance with the prior art. As shown in FIG. 3, micromachine chip 12 and cap 42 are attached to a substrate 52. Bond pads 16 are electrically connected to traces 44 by bond wires 46. To prevent accumulation of static charge on cap 42 which would render micromachine chip 12 inoperable, cap 42 is electrically connected to a ground trace 48 by a bond wire 50. Ground trace 48 is grounded during use. Although effective at prevent accumulation of static charge on cap 42, grounding cap 42 by electrically connecting cap 42 to ground through bond wire 50 and ground trace 48 is relatively labor intensive and complex which increases the cost of fabricating package 40.
In accordance with the present invention, a micromachine package includes a micromachine chip having a micromachine area in an upper surface of the micromachine chip. A bead attached to the upper surface of the micromachine chip and a coupon attached to the bead form an enclosure. The enclosure defines a cavity, i.e., a free space, above the micromachine area. The cavity allows a moveable structure of the micromachine area to freely move.
Of importance, of importance, the coupon has an area substantially less than an area of the upper surface of the micromachine chip. More particularly, the coupon is sized to extend only slightly beyond the micromachine area. In this manner, the amount of material used for the coupon is minimized. By minimizing the amount of material used for the coupon, the costs associated with the micromachine package are also minimized.
Recall that in the prior art, a single lid was attached to a plurality of micromachine chips while still in wafer form. Thus, in the prior art, the cap which covered the micromachine area had an area before trimming equal to the area of the upper surface of the micromachine chip. Accordingly, more material was used for the cap of the prior art than for the coupon in accordance with the present invention. As a result, the micromachine package in accordance with the present invention is fabricated at a lower cost than the micromachine package of the prior art.
To further reduce the costs associated with each micromachine package, in one embodiment, a plurality of coupons are attached to a plurality of micromachine chips while the micromachine chips are still in wafer form. Of importance, the plurality of micromachine chips are tested for validity (i.e., to determine whether each micromachine chip is good or defective) before the coupons are attached and while the plurality of micromachine chips are integrally connected to one another. Any defective micromachine chips are marked or otherwise identified.
Advantageously, the coupons are attached only to the micromachine chips which have been tested and found to be good. The coupons are not attached to defective micromachine chips. In this manner, waste of coupons is avoided and labor associated with attaching the coupons to defective micromachine chips is saved. This, in turn, minimizes the cost associated with the fabrication of each batch of micromachine packages and, more particularly, with each micromachine package.
Recall that in the prior art, a single lid was attached to a plurality of micromachine chips while still in wafer form. Thus, in the prior art, a cap was attached to each micromachine chip even if the micromachine chip was defective. Accordingly, the caps and labor associated with attaching the caps to the defective micromachine chips were wasted in the prior art. As a result, a micromachine package in accordance with the present invention is less expensive to manufacture than a micromachine package of the prior art.
In one embodiment, a method includes attaching a bead to an inner surface of a coupon. The method further includes attaching the bead to an upper surface of a micromachine chip. The bead and the coupon form an enclosure which defines a cavity above a micromachine area in the upper surface of the micromachine chip.
A lower surface of the micromachine chip is attached to an upper surface of a substrate such as a printed circuit board. Formed on the upper surface of the substrate are a plurality of electrically conductive traces. The bond pads of the micromachine chip are electrically connected to these traces by a plurality of bond wires.
In one embodiment, the coupon is ceramic. In accordance with this embodiment, the coupon is not electrically connected to ground, i.e., is electrically floating. Of importance, by forming the coupon of ceramic, the coupon does not accumulate static charge. Accordingly, the prior art requirement of grounding the cap which covered the micromachine area is eliminated. Thus, fabrication of a micromachine package in accordance with the present invention is less labor intensive, less complex and, consequently, less expensive than fabrication of a micromachine package in accordance with the prior art.
In one embodiment, the micromachine package further includes a package body formed around the micromachine chip, the bead and the coupon. Advantageously, the package body is formed at a relatively low cost using a plastic encapsulation process. More particularly, to form the package body, the micromachine chip, the bead, and the coupon are molded in a plastic encapsulant which is injected. Of importance, the enclosure formed by the coupon and the bead has sufficient structural integrity, e.g., strength, to withstand this injection of plastic encapsulant.
In contrast, a micromachine package of the prior art used a cap formed of silicon from a silicon wafer. Disadvantageously, the silicon cap lacked the structural integrity to withstand the injection of plastic encapsulant which occurs during a plastic encapsulation process. Stated another way, the silicon cap would have a tendency to crack if the silicon cap was encapsulated in a plastic encapsulant. Accordingly, if a prior art micromachine package was plastic encapsulated, an unacceptably high incidence of package failure would be observed.
Advantageously, the micromachine package in accordance with this embodiment is a plastic encapsulated package. Accordingly, the micromachine package has superior performance, e.g., resistance to shorting and environmental degradation, than a prior art micromachine package. Further, since the micromachine package is fabricated using a plastic encapsulation process, this superior performance is obtained at a relatively low cost.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.